Conventionally, for an electromechanical transducer for a driving device, a sensor, or the like, a piezoelectric body such as PZT (lead zirconate titanate) is used. Furthermore, in recent years, in response to demands such as for size reduction, an increased packaging density, and cost reduction of apparatuses, there has been an increased use of a MEMS (micro-electro-mechanical systems) device using a Si substrate. In order to apply a piezoelectric body to a MEMS device, it is desirable that the piezoelectric body be reduced in thickness.
By such thickness reduction, high-precision processing using a semiconductor process technology such as film formation or photolithography is enabled, and thus size reduction and an increased packaging density can be realized. Furthermore, devices can be collectively processed so as to achieve a high packaging density on a large-area wafer, and thus cost reduction can be achieved. Moreover, mechanoelectrical transduction efficiency is improved, and thus there are provided advantages such as improvements in property of a driving device and in sensitivity of a sensor.
For example, in a case of a thermal sensor, having a MEMS configuration, it is reduced in thermal conductance, so that a measurement sensitivity thereof can be increased, and in a case of an ink-jet head for a printer, nozzles thereof are provided at an increased packaging density, so that high-definition patterning can be performed.
As a method for forming a film of a piezoelectric body such as PZT on a substrate such as of Si (silicon), there are known chemical methods such as CVD, physical methods such as sputtering and ion plating, and liquid phase growth methods such as a sol-gel method. A film of PZT thus formed exhibits an excellent piezoelectric effect when crystals thereof have a perovskite structure.
A PZT film formed on an electrode on a Si substrate has, due to a difference in lattice constant from that of crystals of the electrode, a polycrystalline structure in which a plurality of crystals assemble together in the form of columns. It is known that the more of such columnar crystals are grown on the same crystal face in a film thickness direction (the higher an orientation characteristic is), the higher a piezoelectric property of the film is.
Recent years have seen a demand for a further improved property of a piezoelectric film such as of PZT. As one measure to obtain such an improved piezoelectric property, an impurity is added so that a relative dielectric constant and a piezoelectric property are improved. It is known that, particularly, (Pb1-XLaX)(ZrYTi1-Y)1-X/4O3 (hereinafter, referred to as PLZT), a substance obtained by substituting Pb located at an A site in a piezoelectric body having a perovskite structure (which is ideally a crystalline structure having a unit cell of a cubic system and composed of a metal A located at each vertex of a cubic crystal, a metal B located at a body center, and oxygen O located at each face center of the cubic crystal, and encompasses distorted cubic crystals such as a tetragonal crystal, an orthorhombic crystal, and a rhombohedral crystal) with La (lanthanum) that is an element having a valence number one number higher than that of Pb has a high relative dielectric constant and a high piezoelectric constant.
For example, a non-patent document 1 discloses that, in PLZT in the form of bulk ceramics, with a predetermined La added amount (for example, 8%) and a Zr/Ti ratio of 60/40 therein, a high piezoelectric property is obtained.
In a case, however, where an attempt is made to obtain a thin film of PLZT by film formation, resulting PLZT in the form of a thin film is poorer in crystallinity as compared with PZT and does not provide such a high property as is obtained when in the form of bulk ceramics.
As a solution to this, a patent document 1 discloses a technique in which a PLT layer free of Zr is formed, and a PLZT layer is formed on the PLT layer. In this technique, by using the PLT layer having good crystallinity as an undercoating layer, crystallinity of PLZT can be improved. Furthermore, a patent document 2 discloses a technique in which, in order to alleviate a lattice mismatch between a buffer layer as a base and a piezoelectric layer main body (PZT), a stepped layer composed of a plurality of layers whose compositions vary in a stepwise manner is provided between them. In this stepped layer, a ratio of Zr/Ti (molar ratio) of each of the layers constituting the stepped layer gradually decreases or increases with increasing distance from the buffer layer in a thickness direction.